Positive electrode active material for lithium secondary battery, comprising lithium cobalt oxide for high voltage, and method for preparing same

ABSTRACT

The present invention provides a positive active material for a rechargeable lithium battery, the active material including a dopant and having a crystalline structure in which metal oxide layers (MO layers) including metals and oxygen and reversible lithium layers are repeatedly stacked, wherein in a lattice configured by oxygen atoms of the MO layers adjacent to each other, the dopant time of charge, thereby forming a lithium trap and/or lithium dumbbell structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 16/070,588filed Jul. 17, 2018, which is a national phase entry under 35 U.S.C. §371 of International Application No. PCT/KR2017/007599, filed on Jul.14, 2017, which claims the benefit of Korean Patent Application No.10-2016-0116951, filed in the Korean Intellectual Property Office onSep. 12, 2016, the disclosures of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a positive electrode active materialfor a rechargeable lithium battery including a lithium cobalt oxide fora high voltage, and a method for preparing same.

BACKGROUND

As technology development and demand for mobile devices have increased,there has been a rapid increase in demand for secondary batteries asenergy sources, and thus a rechargeable lithium battery of secondarybatteries having high energy density and operation potential, a longcycle-life, and a low discharge rate has been commercially available.

In addition, considerable research on an electric vehicle and a hybridelectric vehicle to replace a vehicle using a fossil fuel such as agasoline vehicle, a diesel vehicle, and the like, which are regarded asprimary causes of air pollution, has been undertaken, as interest in theenvironment has recently increased. The electric vehicle, the hybridelectric vehicle, and the like mainly have a power source of a nickelhydrogen metal secondary battery, but researches on utilizing arechargeable lithium battery having a high energy density and dischargehave been actively made, and they are entering the commercializationstage.

Representatively, a prismatic secondary battery and a pouch secondarybattery applicable to a product such as a mobile phone having a thinthickness have been highly demanded in a view of a battery shape, and arechargeable lithium battery such as a lithium ion battery, a lithiumion polymer battery, and the like having a merit of high energy density,discharge, output stability, and the like is highly demanded in a viewof a material.

Currently, LiCoO₂, a ternary system (NMC/NCA), LiMnO₄, LiFePO₄, and thelike are being used as a positive electrode material for therechargeable lithium battery. Among them, LiCoO₂ has problems in thatcobalt is expensive and it has low capacity in the same voltage comparedto the ternary system, so the amount of use of the ternary system andthe like has been gradually increasing to provide a secondary batterywith higher capacity.

However, LiCoO₂ has been mainly used so far since it has excellentproperties such as a high compression density and electrochemicalcharacteristics such as a high cycle characteristic. At the same time,LiCoO₂ has problems in that a charge and discharge current amount is lowat about 150 mAh/g, and a crystalline structure at a voltage of greaterthan or equal to 4.3 V is unstable to decrease a cycle-lifecharacteristic.

Particularly, when applying a high voltage for developing ahigh-capacity secondary battery, a Li amount for LiCoO₂ is increased, soit has problems in that the possibilities of destabilizing a surface anda structure arise to generate a gas due to a side-reaction with theelectrolyte solution, so stability is deteriorated, for example,combustion or a swelling phenomenon, and cycle-life characteristics aredramatically deteriorated.

In order to solve the problems, doping or coating a metal such as Al,Ti, Mg, or Zr on a surface of the LiCoO₂ is a method that is generallyused. However, even in the case of doping the metal, a phase change maybe generated, and furthermore, in the case of a coating layer includingthe metal, it may interrupt Li ion transfer during the charge anddischarge, thereby potentially causing the performance of the secondarybattery to be deteriorated.

Accordingly, it is stably oxidized/reduced only until about 4.45 Vthrough the conventional doping/coating method, so different approachesfrom the conventional art are needed at greater than or equal to 4.5 V.

Thus, there is a great need to develop a lithium cobalt oxide-basedpositive active material which may ensure structural stability withoutdeteriorating performance even at a high voltage.

Technical Problem

The present invention aims to solve the above-described problems of theconventional art and technical problems required from the past.

By repeating in-depth studies and various experiments, the inventors ofthis application confirmed that when lithium cobalt oxide particlesinclude a certain element as a dopant, wherein in a crystallinestructure in which metal oxide layers (MO layers) including metals andoxygen and reversible lithium layers are repeatedly stacked, the dopantand/or lithium ions move from octahedral sites to tetrahedral sites atthe time of charge, thereby forming a lithium trap and/or a lithiumdumbbell structure, which will be described later, a desirable effect isexhibited, and completed the present invention.

Technical Solution

The positive electrode active material for a rechargeable lithiumbattery including the lithium cobalt oxide particles according to thepresent invention includes lithium cobalt oxide particles including atleast one selected from Mg, Nb, Zr, Ti, Mo, and V as a dopant;

the lithium cobalt oxide particles have a crystalline structure in whichmetal oxide layers (MO layers) including metals and oxygen andreversible lithium layers in which lithium ions move reversibly at thetime of charge and discharge are repeatedly stacked; and

the dopant and/or lithium ions move from octahedral sites to tetrahedralsites at the time of charge in a lattice configured by oxygen atoms ofthe MO layers adjacent to each other, thereby forming a lithium trapand/or lithium dumbbell structure.

Generally, when the lithium cobalt oxide is used as a positive activematerial at a high voltage, it causes problems in that the crystalstructure becomes defective while a large amount of lithium ions arereleased from the lithium cobalt oxide particles, such that thedestabilized crystalline structure collapses to deterioratereversibility. In addition, in a state in which the lithium ion isreleased, when Co³⁺ or Co⁴⁺ ions present on the surface of the lithiumcobalt oxide particle are reduced by an electrolyte solution, oxygen isdetached from the crystalline structure to accelerate the structuralcollapse.

Accordingly, in order to stably use the lithium cobalt oxide under ahigh voltage, even if a large amount of lithium ions are emitted, theside reaction of Co ions and the electrolyte solution should besuppressed while the crystalline structure thereof is stably maintained.

Therefore, in the present invention, the lithium cobalt oxide particlesinclude a dopant of Mg, Nb, Zr, Ti, Mo, V, and the like in a structurein which MO layers and reversible lithium layers are repeatedly stacked,and when the dopant and/or lithium ions move from octahedral sites totetrahedral sites at the time of charge, thereby forming a lithium trapand/or a lithium dumbbell structure, a repulsive force among metals ofthe MO layer, lithium ions of the tetrahedral site, and dopants occursto suppress a phenomenon in which the MO layers relatively slide,thereby effectively preventing the structural change.

The lithium trap structure is a structure in which lithium ions aredisposed in tetrahedral sites at the time of charge, in a latticeconfigured by oxygen atoms of the first MO layer and the second MO layeradjacent to each other, and in this case, a repulsive force in avertical direction occurs among metal cations of the second MO layer,lithium ions of the tetrahedral site, and a dopant of the first MOlayer, so as to function as a kind of stopper when the MO layer slidesin a horizontal direction, thereby suppressing the structural change.

Similarly, the lithium dumbbell structure is a structure in whichlithium ions between the first MO layer and the second MO layer are in atetrahedral site at the time of charge, and dopants between the secondMO layer and the third MO layer are in the tetrahedral site, wherein thelithium ions and the dopant are symmetrically positioned on a center ofthe second MO layer in a lattice configured by oxygen atoms of themutually adjacent first MO layer, second MO layer, and third MO layer,and a repulsive force occurs in a vertical direction among the lithiumions at the tetrahedral site between the first MO layer and the secondMO layer, the metal cations of the second MO layer, and the dopants atthe tetrahedral side between the second MO layer and the third MO layer,so as to function as a kind of a stopper when the MO layer is slid in ahorizontal direction, thereby suppressing the structure change.

For understanding the structures, schematic views thereof are shown inFIGS. 4 and 5 .

Referring to FIGS. 4 and 5 , the lithium cobalt oxide has a crystallinestructure in which a MO layer commonly including a metal and oxygen anda reversible lithium layer in which lithium ions reversibly move at thetime of charge and discharge are repeatedly stacked.

First, for describing a structure of the lithium trap, referring to aleft view of FIG. 4 together with FIG. 5 , cobalt ions of a first MOlayer and a second MO layer and lithium ions of a lithium layer aredisposed at all octahedral sites in a lattice region including no dopantin the crystalline structure, but in the lattice region where the cobalt(Co) ions of the first MO layer are substituted with a dopant such asMg, Zr, and the like, the crystalline structure is changed so thatlithium ions of the lithium layer disposed between the first MO layerand the second MO layer and the dopant of the first MO layer move totetrahedral sites, which is called a lithium trap structure. Whenobtaining the structure, when sliding in a horizontal direction in thecrystalline structure of the lithium cobalt oxide, the lithium ions ofthe lithium layer and the Co ions of the MO layers are disposed in adiagonal line in a region including no dopant, so there is no increaseof the internal energy, but in a region including the dopant and formingthe lithium trap structure, the lithium ions of the lithium layer, Coions of the second MO layer, and dopant of the first MO layer are alldisposed in a straight line to generate a repulsive force among them, soaccompanying the internal energy increase to maintain the structure, andthereby the sliding is prevented by the characteristics that a materialis to exist in the most stable state which has the lowest internalenergy.

Similarly, the lithium dumbbell structure shown in a right view of FIG.4 has the same fundamental forming protocol as in the lithium trapstructure, but lithium ions between the first MO layer and the second MOlayer and dopants between the second MO layer and the third MO layermove to tetrahedral sites, meaning a structure in which the lithium ionsand the dopants are symmetrically disposed to each other in the centerof the second MO layer.

In this case, in order to clearly describe the tetrahedral site and theoctahedral site, the schematic view thereof is shown in FIG. 1 .

Referring to FIG. 1 , the tetrahedral site (hole) refers to a spaceformed in a center of an atom cluster when a single atom is contactedwith three atoms of an atomic layer disposed below the same, so twotetrahedral sites exist per atom when atoms having the same size arepositioned as near as possible, and the octahedral site (hole) refers toa space formed in a cluster of six atoms for an octahedron which isformed when each of three atoms for two pairs of triangles contact withan alignment angle of 60 degree, wherein the space is larger than thetetrahedral site, and one octahedral site is present per atom when theatoms having the same size are disposed as near as possible.

Accordingly, both the lithium dumbbell structure and the lithium trapstructure may suppress the structural change by generating the repulsiveforce among the metal cations, dopants, and lithium ions in a verticaldirection, but in a case of the lithium dumbbell structure, a trapstructure is formed not only between the first MO layer and the secondMO layer but also between the second MO layer and the third MO layer,unlike the lithium trap structure, so it is more preferably since itprovides much better structural stability effects than the lithium trapstructure generating the repulsive force between first MO layer and thesecond MO layer.

The dopant which may form the lithium trap or the lithium dumbbellstructure may be included at 0.001 to 1 wt % based on the cobalt amountof the lithium cobalt oxide particle, more particularly, the dopantamount may be included at 0.01 wt % to 0.3 wt %, specifically, 0.02 wt %to 0.2 wt %, and more specifically, 0.02 wt % to 0.1 wt %, to provide anactive material with excellent capacity and energy by improving thesufficiently high voltage stability and also maintaining the high Coamount.

When the amount of dopant is less than 0.001 wt % which is out of therange, the ratio of the dopant in the positive active material particleis too low to obtain the described structure, so the effect of improvingthe structural stability of the active material is almost nothing, andon the other hand, when is greater than 1 wt %, the ratio of the dopantin the positive active material particle is excessively high, evencausing deterioration of the lithium transfer, so it has problems inthat the output characteristic is deteriorated, and the overall capacityof the positive active material is relatively decreased.

Meanwhile, the lithium trap structure and the lithium dumbbell structuremay be formed differently depending upon a kind of a predetermineddopant.

In one specific example, the dopant includes at least one selected fromMg, Nb, Zr, and V, and may form a lithium trap and a lithium dumbbellstructure.

In addition, the dopant includes at least one selected from Mg, Nb, Zr,Ti, Mo, and V, and may form a lithium trap structure.

The forming of the structure according to a kind of the dopant as statedabove is further described in detail in the following ExperimentalExample 1.

From the results as stated above, in order to further improve thestructural stability of the lithium cobalt oxide, it is more preferablefor the lithium cobalt oxide particle according to the present inventionto include at least one selected from a group consisting of Mg, Nb, Zr,and V as a dopant, and more specifically, the dopant may be Mg and/orZr.

Meanwhile, in the positive active material for a secondary battery cellaccording to the present invention, when r refers to an average radiusof the lithium cobalt oxide particle, the dopant concentration of anouter bulk of a particle surface to 0.9*r may be relatively higher thanthe dopant concentration of an inner bulk from 0.9*r to a particlecenter.

In one example, the positive active material according to the presentinvention may include Mg as a dopant included in the lithium cobaltoxide particle, and the Mg concentration of the outer bulk may berelatively higher than the Mg concentration in the inner bulk.

In another example, the positive active material according to thepresent invention may include Mg and Zr as the dopant included in thelithium cobalt oxide particle, where Zr is primarily included in theouter bulk, and Mg is mostly included in the inner bulk, and the Zrconcentration of the outer bulk is relatively higher than the Mgconcentration of the inner bulk.

Thus when the positive active material for a secondary battery accordingto the present invention is prepared by the following method, when thedopant concentration of the outer bulk of the particle surface of thelithium cobalt oxide to 0.9*r is formed relatively higher than thedopant concentration of the inner bulk of 0.9*r to the particle center,an appropriate ratio of the dopants included in the outer bulk and theinner bulk may effectively prevent the sliding phenomenon of the MOlayers on the surface even under the charge condition of greater than orequal to 4.5 V, thereby further improving stability of the positiveactive material.

At a high voltage of greater than or equal to 4.5 V, almost no Liremains on the surface of LiCoO₂, so it is very easy to convert it to an01 phase. Accordingly, the surface should be strengthened to prevent aphase transition, and it is therefore necessary to set the dopantconcentration in the inner bulk to be higher than the dopantconcentration of the outer bulk.

When the medium particle size (D50) is too small, it is not preferablesince it is difficult to fabricate a particle, and the crystallinestructure according to the present invention may be incomplete, and onthe other hand, when it is too large, it undesirably has a poorcompression density of the electrode including particles.

Accordingly, the lithium cobalt oxide particle according to the presentinvention may have a medium particle size (D50) of 5 micrometers to 25micrometers, particularly, 10 micrometers to 25 micrometers, and moreparticularly, 15 micrometers to 20 micrometers.

It may further include at least one selected from a group consisting ofCa, Al, Sb, Si, and Zn, and more particularly, at least one metalselected from a group consisting of Ca, Al, and Sb, in order tostabilize the crystalline structure, in addition to the dopant.

The lithium cobalt oxide particle may be further coated by protectivechemicals, wherein the protective chemicals may be at least one of ametal, an oxide, a phosphate, and a fluoride, and the metal may be atleast one of Mg, Nb, Zr, Ti, Mo, V, Zn, Si, and Al.

The protective chemicals may be coated on a lithium cobalt oxideparticle using solution reaction, mechanical grinding, solid reaction,and the like, and may reduce a speed of reacting an electrolyte in theseparator with the lithium cobalt oxide particle during the charge anddischarge of a battery cell, so as to suppress expansion or capacityloss caused by the reaction of the positive electrode active coating.

In this case, the amount of the protective chemical coated on thelithium cobalt oxide particle may be 0.02 wt % to 0.8 wt %, and thethickness may be 30 nm to 250 nm. When is included at less than 0.02 wt% or formed in a thickness of less than 30 nm, it may not obtain thedesirable effects, and when is included at greater than 0.8 wt % orformed in a thickness of greater than 250 nm, it is not preferable sincethe output characteristics of the battery are deteriorated.

According to the findings of the inventors of the present invention, itis confirmed that the lithium cobalt oxide particle according to thepresent invention has excellent chemical stability when the amount ofthe protective chemicals is 0.02 wt % to 0.6 wt %, particularly, 0.03 wt% to 0.4 wt %, and more particularly, 0.04 wt % to 0.1 wt %.

In addition, when the protective chemicals have a thickness of 30 nm to200 nm, particularly, 30 nm to 185 nm, and more particularly, 30 nm to150 nm, desirable output characteristics are confirmed.

The present invention provides a method of preparing a positive activematerial for a secondary battery, and the method includes:

(a) mixing a cobalt precursor, a lithium precursor, and a first dopingprecursor and then firing the same to synthesize a first dopingparticle;

(b) coating a second doping precursor on a surface of the first dopingparticle; and

(c) heating the coated first doping particle to synthesize a seconddoping particle which is a lithium cobalt oxide particle.

That is, the lithium cobalt oxide particle according to the presentinvention may be obtained through two sequentially performed dopingprocesses, and specifically, the lithium cobalt oxide particle accordingto the present invention may be obtained by processes of mixing anappropriate amount of the first doping precursor in the precursor stateand firing the same to provide a lithium cobalt-based oxide particlesubstituted with a dopant, coating a salt including an appropriateamount of the dopant on a surface of the first doping particle to coat asecond doping precursor on the surface of the first doping particle, andthen heat treating the same to provide a second doping particle.

The method includes mixing a cobalt precursor with a lithium precursorand a doping precursor under an air atmosphere and primarily firing thesame, and then coating a second doping precursor and secondarily firingthe same to synthesize a lithium cobalt oxide particle, so it maymaintain a higher dopant concentration on the surface compared to thecase of mixing the cobalt precursor, the lithium precursor, and thedoping precursor and firing the same only one time to synthesize alithium cobalt oxide particle, and it may be synthesized in a state inwhich dopants are uniformly dispersed even to the inside of the particlecompared to the case of synthesizing a undoped lithium cobalt oxideparticle and then coating the doping precursor and firing the same, soit is more preferable in a view of imparting the desirable effects.

Herein, the cobalt precursor is a cobalt oxide, and the kind of thecobalt oxide used in the method according to the present invention isnot limited, but particularly, may be at least one selected from thegroup consisting of CO₃O₄, CoCO₃, Co(NO₃)₂, and Co(OH)₂.

In addition, the lithium precursor is not limited as long as thecompound includes a lithium source, particularly, at least one selectedfrom the group consisting of Li₂CO₃, LiGH, LiNO₃, CH₃COOLi, andLi₂(COO)₂.

The doping precursor may be at least one selected from the groupconsisting of a metal, a metal oxide, or a metal salt for the dopants,and the first doping precursor and the second doping precursor arecomposed of a metal, a metal oxide, or a metal salt including at leastone independently selected from the group consisting of Mg, Nb, Zr, Ti,Mo, and V. In one specific example, the first doping precursor and thesecond doping precursor may be the same or different.

But when the first doping precursor is the same as the second dopingprecursor, it may show excellent results in both views of the insidedoping effect and the surface doping effect. In other words, when thefirst doping particle particularly enhances the structural safety, thesecond doping particle including the same elements as in the firstdoping particle may also provide excellent surface safety. Thus it ismore preferable than the case of including different dopants.

Meanwhile, an amount of the dopant doped from the second dopingprecursor may be greater than or equal to the amount of the dopant dopedfrom the first doping precursor.

Herein, the first doping particle generated from the first dopingprecursor is mainly disposed in the inner bulk of 0.9*r to the particlecenter, and the second doping particle generated from the second dopingprecursor is generally disposed in the outer bulk of the particlesurface to 0.9*r, so it may be formed so that the dopant concentrationof the outer bulk of the lithium cobalt oxide is relatively higher thanthe dopant concentration of the inner bulk.

As the structural change of the layered positive electrode material suchas a lithium cobalt oxide at a high voltage of greater than or equal to4.5 V occurs from the surface, the surface of the lithium cobalt oxideparticle, which is the outer bulk, is formed at a higher concentrationthan the inner bulk to more strongly suppress the structural change ofthe surface, thereby providing effects of further improving thestability.

Meanwhile, the firing of the process (a) may be performed at 900° C. to1100° C. for 8 hours to 12 hours, and the heat treatment of the process(c) may be performed at 700° C. to 900° C. for 1 hour to 6 hours.

When the process (a) and the process (c) are performed out of theranges, for example, performed at an excessively low temperature or foran excessively short time, the inside structure and the surfacestructure of the positive active material particle are unstably formedand unfavorably doped, and on the other hand, when the process (a) andthe process (c) are performed out of the ranges, for example, performedat an excessively high temperature or for an excessively long time, thephysical and chemical characteristics of the lithium cobalt-based oxidefor the positive active material particle are changed, even unfavorablycausing performance deterioration.

The present invention provides a positive electrode manufactured bycoating a slurry including the positive active material for a secondarybattery, a conductive material, and a binder on a current collector.

The positive electrode may be manufactured by, for example, coating apositive electrode mixture including the positive active material, aconductive material, and a binder, on a positive electrode currentcollector, and drying it, and if required, the positive electrodemixture may further include a filler.

The positive electrode current collector is generally made at athickness of 3 to 500 μm, but this is not particularly limited as longas it does not cause a chemical change in the battery and has highconductivity, and for example, one selected from stainless steel,aluminum, nickel, titanium, and aluminum, or stainless steel of whichthe surface is treated with carbon, nickel, titanium, or silver, may beused, and specifically, aluminum may be used. The current collector mayhave fine irregularities formed on a surface thereof to increaseadhesive force of the cathode active material, and various forms such asa film, a sheet, a foil, a net, a porous body, foam, a non-woven fabricbody, and the like, are possibly used.

The positive active material may include, for example: a layeredcompound such as a lithium nickel oxide (LiNiO₂) layered compound 1 or acompound substituted with one or more transition metals; a lithiummanganese oxide such as one of a chemical formula Li_(1+x)Mn_(2-x)O₄(wherein x is 0 to 0.33), LiMnO₃, LiMn₂O₃, LiMnO₂, and the like; alithium copper oxide (Li₂CuO₂); a vanadium oxide such as LiV₃O₈, LiV₃O₄,V₂O₅, Cu₂V₂O₇, and the like; a Ni site-type lithium nickel oxiderepresented by a chemical formula LiNi_(1-x)M_(x)O₂ (wherein M=Co, Mn,Al, Cu, Fe, Mg, B, or Ga, and x=0.01 to 0.3); a lithium manganesecomposite oxide represented by a chemical formula LiMn_(2-x)M_(x)O₂(wherein M=Co, Ni, Fe, Cr, Zn, or Ta, and x=0.01 to 0.1) or Li₂Mn₃MO₈(wherein M=Fe, Co, Ni, Cu, or Zn); LiMn₂O₄ in which a part of Li in thechemical formula is substituted with an alkaline earth metal ion; adisulfide compound; Fe₂(MoO₄)₃; and the like, in addition to the lithiumcobalt oxide particle, but is not limited thereto.

The conductive material is generally added in an amount of 1 to 30 wt %based on the total weight of the mixture including the positive activematerial. Such a conductive material is not particularly limited as longas it has conductivity without causing a chemical change in the battery,and examples thereof may include: graphite such as natural graphite andartificial graphite; carbon black such as carbon black, acetylene black,Ketjen black, channel black, furnace black, lamp black, and summerblack; conductive fibers such as carbon fiber and metal fiber; metalpowders such as carbon fluoride, aluminum, and nickel powder; conductivewhiskers such as zinc oxide and potassium titanate; conductive metaloxides such as titanium oxide; conductive materials such aspolyphenylene derivatives; and the like.

The binder included in the positive electrode is a component thatassists in bonding between the active material and the conductivematerial and bonding to the current collector, and is generally added inan amount of 1 to 30 wt % based on the total weight of the mixtureincluding the cathode active material. Examples of the binder mayinclude polyvinylidene fluoride, polyvinyl alcohol, carboxylmethylcellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose,polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,an ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM,styrene-butadiene rubber, a fluorine rubber, various copolymers, and thelike.

The present invention also provides a secondary battery characterized byincluding the positive electrode and the negative electrode, and anelectrolyte solution. The secondary battery is not particularly limited,but specific examples thereof include a rechargeable lithium batterysuch as a lithium ion battery and a lithium ion polymer battery havingadvantages such as high energy density, discharge voltage, and outputstability.

Generally, the rechargeable lithium battery includes a positiveelectrode, a negative electrode, a separator, and a lithiumsalt-containing non-aqueous electrolyte solution.

Hereinafter, other aspects of the rechargeable lithium battery aredescribed.

The negative electrode is manufactured by coating and drying thenegative active material on the negative current collector, andoptionally, may further include components as described above as needed.

The negative current collector is generally made in a thickness of 3 to500 micrometers. Such a negative current collector is not particularlylimited as long as it has conductivity without causing a chemical changein the battery, and for example, copper, stainless steel, aluminum,nickel, titanium, fired carbon, or copper, or stainless steel of whichthe surface is treated with carbon, nickel, titanium, silver, and thelike, an aluminum-cadmium alloy, or the like, may be used. Further, likethe positive electrode current collector, the binding force of thenegative active material may be strengthened by forming fineirregularities on the surface thereof, and various forms such as a film,a sheet, a foil, nets, a porous body, foam, and a non-woven fabric bodymay be used.

The negative active material may include, for example: carbon such ashard carbon and graphite-based carbon; a metal composite oxide such asLi_(x)Fe₂O₃ (0≤x≤1), Li_(x)WO₂ (0≤x≤1), and Sn_(x)Me_(1-x)Me′_(y)O_(z)(Me: Mn, Fe, Pb, Ge; Me′: Al, B, P, Si, Groups 1, 2, and 3 elements ofthe periodic table, a halogen; 0<x≤1; 1≤y≤3; 1≤z≤8); a lithium metal; alithium alloy; a silicon-based alloy; tin-based alloy; metal oxides suchas SnO, SnO₂, PbO, PbO₂, Pb₂O₃, Pb₃O₄, Sb₂O₃, Sb₂O₄, Sb₂O₅, GeO, GeO₂,Bi₂O₃, Bi₂O₄, and Bi₂O₅; a conductive polymer such as polyacetylene;Li—Co—Ni-based materials; and the like.

The separator is interposed between the positive electrode and thenegative electrode, and an insulating thin film having high ionpermeability and mechanical strength is used. The separator has a porediameter of generally 0.01 to 10 μm, and a thickness of 5 to 300 μm. Forthe separator, for example, an olefin-based polymer such aspolypropylene, which is chemically resistant and hydrophobic, or thelike, or a sheet or a nonwoven fabric formed of a glass fiber,polyethylene, or the like, is used. In the case that a solid electrolytesuch as a polymer is used as the electrolyte, the solid electrolyte mayalso serve as the separator.

The lithium salt-containing non-aqueous electrolyte solution is composedof a non-aqueous electrolyte solution and a lithium salt. Thenon-aqueous electrolyte solution may include a non-aqueous organicsolvent, an organic solid electrolyte, an inorganic solid electrolyte,and the like, but is not limited thereto.

The non-aqueous organic solvent may include, for example, aproticorganic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate,ethylene carbonate, butylene carbonate, dimethyl carbonate, diethylcarbonate, γ-butyrolactone, 1,2-dimethoxy ethane, tetrahydroxy Franc,2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide,dimethylformamide, dioxolane, acetonitrile, nitromethane, methylformate, methyl acetate, phosphate triester, trimethoxy methane,dioxolane derivative, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, a propylene carbonate derivative, atetrahydrofuran derivative, ether, methyl propionate, and ethylpropionate.

The organic solid electrolyte may include, for example, polyethylenederivatives, polyethylene oxide derivatives, polypropylene oxidederivatives, phosphate ester polymers, poly agitation lysine, polyestersulfide, polyvinyl alcohol, polyfluorovinylidene, polymers including anionic dissociation group, and the like.

The inorganic solid electrolyte may include, for example, nitrides,halides, sulfates, or the like of Li such as Li₃N, LiI, Li₅NI₂,Li₃N—LiI—LiOH, LiSiO₄, LiSiO₄—LiI-LiGH, Li₂SiS₃, Li₄SiO₄,Li₄SiO₄—LiI—LiOH, Li₃PO₄—Li₂S—SiS₂, and the like.

The lithium salt is a material which is readily soluble in thenon-aqueous electrolyte, and for example, LiCl, LiBr, LiI, LiClO₄,LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄,CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi, lithium chloroborane, lower aliphaticlithium carbonates, 4 phenyl lithium borate, imides, and the like may beused.

Further, to the non-aqueous electrolyte solution, for the purpose ofimproving charge and discharge characteristics, flame retardancy and thelike, for example, pyridine, triethylphosphite, triethanolamine, cyclicether, ethylene diamine, n-glyme, triamide hexaphosphate, nitrobenzenederivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone,N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammoniumsalts, pyrrole, 2-methoxy ethanol, aluminum trichloride, and the likemay be added. In some cases, for imparting incombustibility, ahalogen-containing solvent such as carbon tetrachloride and ethylenetrifluoride may be further included, and for improving high temperaturestorage characteristics, carbon dioxide gas may be further included, andFEC (fluoro-ethylene carbonate), PRS (propene sultone), and the like maybe further included.

The present invention provides a battery pack including the secondarybattery and a device including the battery pack, wherein the batterypack and the device are known in the art, so the detailed descriptionsare omitted in the present specification.

The device may be, for example, a laptop computer, a netbook, a tabletPC, a mobile phone, an MP3, a wearable electronic device, a power tool,an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-inhybrid electric vehicle (PHEV), an electric bicycle (E-bike), anelectric scooter (E-scooter), an electric golf cart, or a system forelectric power storage, but is not limited thereto.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an octahedral site or a tetrahedralsite where a dopant or a lithium ion is disposed.

FIG. 2 is a graph showing generated energy of a lithium dumbbellstructure according to Experimental Example 1 of the present invention.

FIG. 3 is a graph showing generated energy of a lithium trap structureaccording to Experimental Example 1 of the present invention.

FIG. 4 is a schematic view showing a lithium dumbbell and a lithium trapstructure according to an embodiment of the present invention.

FIG. 5 is a side-sectional view of a dopant or a lithium ion in anoctahedral site or a tetrahedral site according to an embodiment of thepresent invention.

FIG. 6 is a graph showing capacity retention of each battery of Examples1 and 2 according to experimental examples of the present invention anda comparative example depending upon a cycle number.

MODE FOR INVENTION

Hereinafter, exemplary embodiments of the present invention will bedescribed referring to drawings, but this is for easier understanding ofthe present invention, and the scope of the present invention is notlimited thereto.

Preparation Example 1

8.19 g of Co₃O₄ and 3.74 g of Li₂CO₃ were mixed and then fired in afurnace at 1000° C. for 10 hours to provide a lithium cobalt oxide.

Preparation Example 2

8.19 g of Co₃O₄, 3.74 g of Li₂CO₃, and 1000 ppm of Al (Al source: Al₂O₃)were mixed and then fired in a furnace at 1000° C. for 10 hours toprovide a lithium cobalt oxide in which Al of the lithium cobalt oxidewas doped.

Preparation Example 3

A lithium cobalt oxide was obtained in accordance with the sameprocedure as in Preparation Example 2, except that Mg (Mg source: MgO)was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 4

A lithium cobalt oxide was obtained in accordance with the sameprocedure as in Preparation Example 2, except that Ti (Ti source: TiO₂)was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 5

A lithium cobalt oxide was obtained in accordance with the sameprocedure as in Preparation Example 2, except that Zr (Zr source: ZrO₂)was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 6

A lithium cobalt oxide was obtained in accordance with the sameprocedure as in Preparation Example 2, except that Nb (Nb source: Nb₂O₅)was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 7

A lithium cobalt oxide was obtained in accordance with the sameprocedure as in Preparation Example 2, except that Ta (Ta source: Ta₂O₅)was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 8

A lithium cobalt oxide was obtained in accordance with the sameprocedure as in Preparation Example 2, except that Mo (Mo source: MoO₃)was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 9

A lithium cobalt oxide was obtained in accordance with the sameprocedure as in Preparation Example 2, except that W (W source: WO₃) wasused as a dopant instead of the Al in Preparation Example 2.

Preparation Example 10

A lithium cobalt oxide was obtained in accordance with the sameprocedure as in Preparation Example 2, except that V (V source: V₂O₅)was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 11

A lithium cobalt oxide was obtained in accordance with the sameprocedure as in Preparation Example 2, except that Mn (Mn source: MnO₂)was used as a dopant instead of the Al in Preparation Example 2.

Experimental Example 1

The lithium cobalt oxides obtained from Preparation Examples 1 to 11were used as a positive active material, PVdF was used as a binder, andnatural graphite was used as a conductive material. The positive activematerial:the binder:the conductive material were mixed at a weight ratioof 96:2:2 into NMP and then coated on an Al foil having a thickness of20 μm and dried at 130° C. to provide a positive electrode. Using alithium foil as a negative electrode and an electrolyte solution inwhich 1M of LiPF₆ was dissolved in a solvent of EC:DMC:DEC=1:2:1, a halfcoin cell was fabricated.

While the obtained coin cells were charged at 1.0 C to 4.48 V, generatedenergy producing a lithium trap structure or dumbbell structure wasmeasured and is shown in FIGS. 2 and 3 , and while the obtained coincells were charged at 1.0 C to 4.60 V, generated energy producing alithium trap structure or dumbbell structure was measured and is shownin FIGS. 2 and 3 .

Referring to FIG. 2 , it is understood that when the lithium cobaltoxide particle includes Mg, Zr, Nb, or V as a dopant, the generatedenergy is a negative value, so the lithium dumbbell structure isspontaneously formed under a high voltage of 4.6 V.

In addition, referring to FIG. 3 , when the lithium cobalt oxideparticle includes Mg, Ti, Zr, Nb, Mo, or V as a dopant, the generatedenergy is a negative value, so it is understood that the lithium trapstructure is spontaneously formed under a high voltage of 4.6 V.

Herein, referring to FIGS. 2 and 3 , it is more preferable that Mg, Zr,Nb, or V among dopants have a negative value of the generated energy ofthe lithium trap structure and the dumbbell structure under the highvoltage of 4.6 V. Particularly, Mg and Zr have the greatest negativevalue in the lithium dumbbell structure and the lithium trap structure,respectively, so it is understood that they are the most preferableelements for providing the structures.

The structure generation energy is calculated using VASP (Vienna Abinitio Simulation Program), the DFT functional uses PBE, and thePseudo-potential uses PAW-PBE. In addition, the cut-off energy iscalculated at 500 eV.

Example 1

8.19 g of Co₃O₄, 3.74 g of Li₂CO₃, and 400 ppm of Mg (Mg source: MgO)were mixed to provide 0.04 wt % of Mg based on a total weight of thelithium cobalt oxide particles, and then primarily fired in a furnace at1000° C. for 10 hours to provide a lithium cobalt oxide in which Mg wasdoped in the inner bulk of the lithium cobalt oxide at a concentrationof 400 ppm. Then, in order to provide a coating layer on the obtainedlithium cobalt oxide, a salt including 600 ppm of Zr, which was 15 times(0.06 wt %) the doped Mg amount, was dry-mixed with lithium cobalt oxideparticles and coated, and secondarily fired in a furnace at 800° C. for4 hours to provide a positive active material in which Zr was doped onthe outer bulk at a concentration of 600 ppm.

Example 2

8.19 g of Co₃O₄, 3.74 g of Li₂CO₃, and 600 ppm of Mg (Mg source: MgO)were mixed to provide 0.06 wt % of Mg based on a total weight of thelithium cobalt oxide particles, and then primarily fired in a furnace at1000° C. for 10 hours to provide a lithium cobalt oxide in which Mg wasdoped in the inner bulk of the lithium cobalt oxide at a concentrationof 600 ppm. In order to provide a coating layer on the obtained lithiumcobalt oxide, a salt including 400 ppm of Mg, which was 0.66 times (0.04wt %) the doped Mg amount, was dry-mixed with lithium cobalt oxideparticles and coated, and then subjected to secondary firing in afurnace at 800° C. for 4 hours to provide a positive active material inwhich Mg was doped in the outer bulk at a concentration of 400 ppm.

Example 3

8.19 g of Co₃O₄, 3.74 g of Li₂CO₃, and 600 ppm of Mg were mixed toprovide 0.06 wt % of Mg based on a total weight of the lithium cobaltoxide particle, and subjected to primary firing in a furnace at 1000° C.for 10 hours to provide a lithium cobalt oxide in which Mg was doped inthe inner bulk of the lithium cobalt oxide at a concentration of 600ppm. In order to provide a coating layer on the obtained lithium cobaltoxide, a salt including 400 ppm of Zr, which was 0.66 times (0.04 wt %)the doped Mg amount, was dry-mixed with lithium cobalt oxide particlesand coated, and then subjected to secondary firing in a furnace at 800°C. for 4 hours to provide a positive active material in which Zr wasdoped in the outer bulk at a concentration of 400 ppm.

Example 4

8.19 g of Co₃O₄, 3.74 g of Li₂CO₃, and 400 ppm of Mg were mixed toprovide Mg at 0.04 wt % based on a total weight of the lithium cobaltoxide particles, and then subjected to primary firing in a furnace at1000° C. for 10 hours to provide a lithium cobalt oxide in which Mg wasdoped in the inner bulk of the lithium cobalt oxide at a concentrationof 400 ppm. In order to provide a coating layer on the obtained lithiumcobalt oxide, a salt including 600 ppm of Mg, which was 1.5 times (0.06wt %) the doped Mg amount, was dry-mixed with lithium cobalt oxideparticles and coated, and then subjected to secondary firing in afurnace at 800° C. for 4 hours to provide a positive active material inwhich Mg was doped in the outer bulk at a concentration of 400 ppm.

Comparative Example 1

The lithium cobalt oxide obtained from Preparation Example 3 was used asa positive active material.

Experimental Example 2

Each positive active material particle obtained from Examples 1 and 2and Comparative Example 1, a binder of PVdF, and a conductive materialof natural graphite were used. The positive active material:thebinder:the conductive material were well mixed into NMP to provide aweight ratio of 96:2:2 and coated on an Al foil having a thickness of 20μm and then dried at 130° C. to provide a positive electrode. As anegative electrode, a lithium foil was used, and an electrolyte solutionin which 1M of LiPF₆ was dissolved in a solvent of EC:DMC:DEC=1:2:1 wasused to provide a half coin cell.

The obtained half coin cell was subjected to 30 cycles at 45° C. with anupper limit voltage of 4.5 V, and capacity retention was measured. Theresults are shown in the following Table 1 and FIG. 6 .

TABLE 1 Example Example Example Example Comparative 1 2 3 4 Example 1Capacity 97.8% 91.3% 90.5% 96.7% 78.6% retention (%)

Referring to Table 1, it is confirmed that Examples 1 to 4 maintainedhigh performance with capacity retention of greater than or equal to 90%even after 30 cycles and even under a high voltage condition of 4.5 V,compared to the case of using the undoped lithium cobalt oxide accordingto Comparative Example 1.

This is because the dopants included in the outer bulk of the lithiumcobalt oxide particle suppressed the crystal structure collapse from theexternal surface, and the dopants included in the inner bulk suppresseda side reaction between the electrolyte solution and Co⁴⁺ ions presenton the particle surface in a state of discharging lithium ions, so as tostably maintain the crystal structure.

Meanwhile, as the concentration of dopants included in the outer bulk ishigher than the concentration of the dopants included in the inner bulkin the cases of Examples 1 and 4, the surface stability may be furtherenhanced under the charge condition of greater than or equal to 4.5 V,compared to the cases of Examples 2 and 3, so it is confirmed that ithas much better capacity retention of greater than or equal to 95% afterthe 30 cycles.

In addition, compared to Examples 1 with 4, it is confirmed that thecase of doping the same elements in both the inner bulk and the outerbulk may have much better effects than the case of doping differentelements.

Although description has been given with reference to an exemplaryembodiment of the present invention, a person skilled in the artpertaining to the present invention may variously apply and modify thesame within a scope of the present invention on the basis of thedescription.

As described above, the positive active material particle according tothe present invention includes a predetermined element as a dopant, andhas a crystalline structure in which metal oxide layers (MO layers)including metals and oxygen and reversible lithium layers are repeatedlystacked, and the dopant and/or lithium ions move from octahedral sitesto tetrahedral sites at the time of charge, thereby forming a lithiumtrap and/or a lithium dumbbell structure, thereby providing effects ofsuppressing the structural change on the particle surface to improve thecycle-life characteristics at a high temperature and also of stablymaintaining the crystalline structure even when emitting a large amountof lithium ions.

The invention claimed is:
 1. A positive active material for arechargeable lithium battery comprising lithium cobalt oxide particles,wherein the lithium cobalt oxide particles having dopants, wherein thedopants are Mg and Zr, wherein Zr is primarily included in the outerbulk, and Mg is mainly included in the inner bulk, and the Zrconcentration of the outer bulk of a particle surface to 0.9*r isrelatively higher than the Mg concentration of the inner bulk of 0.9*rto a particle center; the lithium cobalt oxide particles have acrystalline structure in which metal oxide layers (MO layers) includingmetals and oxygen and reversible lithium layers in which lithium ionsmove reversibly at the time of charge and discharge are repeatedlystacked, and the dopant and/or lithium ions move from octahedral sitesto tetrahedral sites at the time of charge in a lattice configured byoxygen atoms of the MO layers adjacent to each other, thereby forming alithium trap and/or a lithium dumbbell structure and providingstructural stability at a high voltage of 4.5 V or greater.
 2. Thepositive active material of claim 1, wherein the dopant is included inan amount of 0.001 to 1 wt % based on a total weight of the lithiumcobalt oxide particles.
 3. The positive active material of claim 1,wherein the lithium trap structure is a structure in which the lithiumions are disposed in tetrahedral sites at the time of charge in alattice configured by oxygen atoms of a first MO layer and a second MOlayer adjacent to each other.
 4. The positive active material of claim1, wherein the lithium dumbbell structure is a structure in whichlithium ions are disposed in tetrahedral sites between a first MO layerand a second MO layer, and the dopants are disposed in tetrahedral sitesbetween the second MO layer and a third MO layer at the time of chargein a lattice configured by oxygen atoms of the first MO layer, thesecond MO, and the third MO layer, so the lithium ions and the dopantsare symmetrically disposed in a center of the second MO layer.
 5. Thepositive active material of claim 1, wherein the positive activematerial comprises the lithium trap and the lithium dumbbell structure.6. The positive active material of claim 1, wherein the positive activematerial comprises the lithium trap structure.
 7. The positive activematerial of claim 1, wherein the lithium trap or the lithium dumbbellstructure suppresses a phenomenon of relatively sliding of the MO layersby a repulsive force generated among metals of the MO layer, lithium ofthe tetrahedral site, and the dopants which are each cations, thussuppressing a structural change.
 8. The positive active material ofclaim 1, wherein the lithium cobalt oxide particles further comprise atleast one selected from the group consisting of Ca, Al, and Sb as adopant.
 9. The positive active material of claim 1, wherein the lithiumcobalt oxide particles are further coated with protective chemicals, andthe protective chemicals are at least one of a metal, an oxide, aphosphate salt, and a fluoride.
 10. The positive active material ofclaim 9, wherein the protective chemicals are included at 0.02 wt % to0.8 wt %.
 11. The positive active material of claim 9, wherein theprotective chemicals have a thickness of 30 nm to 250 nm.
 12. Thepositive active material of claim 1, wherein the lithium cobalt oxideparticles have a medium particle size (D50) of 5 micrometers to 25micrometers.
 13. A method of fabricating lithium cobalt oxide particlesof a positive active material of claim 1, comprising: (a) mixing acobalt precursor, a lithium precursor, and a Mg doping precursor andthen firing the same to synthesize first doping particles; (b) coating aZr doping precursor on a surface of the Mg doping particles; and (c)heat treating the coated Mg doping particles to synthesize Zr dopingparticles which are lithium cobalt oxide particles, wherein an amount ofthe dopant doped from the Zr doping precursor is greater than or equalto an amount of the dopant doped from the Mg doping precursor.
 14. Themethod of claim 13, wherein the Mg doping precursor and the Zr dopingprecursor are independently a metal, a metal oxide, or a metal saltincluding Mg or Zr.
 15. The method of claim 13, wherein the firing ofthe process (a) is performed at 900° C. to 1100° C. for 8 hours to 12hours.
 16. The method of claim 13, wherein the heat treatment of theprocess (c) is performed at 700° C. to 900° C. for 1 hour to 6 hours.